The present invention broadly relates to semiconductor manufacturing operations, and deals more particularly with a method and apparatus for detecting the presence of etchant residue, and especially nitride residue on semiconductor wafers following an etching operation.
An important process in the fabrication of integrated circuits is the removal of various layers of materials formed on a silicon wafer. Two major etching techniques are in common usage. One of these techniques is referred to as wet or chemical etching, wherein a photoresist patterned silicon wafer is immersed in a chemical solution. The other of such techniques is referred to as a dry or plasma etching, wherein a wafer is exposed to a plasma containing a gas.
Plasma etch processes and apparatus are generally well known for etching materials for semiconductor device fabrication. The process begins with application of a masking material, such as photoresist, to a silicon wafer. The masking pattern protects areas of the wafer from the etch process. The wafer is then placed in a plasma reactor or xe2x80x9cetcherxe2x80x9d and it is then etched. Subsequent steps are determined by the type of device being fabricated. This process is especially valuable for the definition of small geometries.
Plasma etching is basically anisotropic and eliminates undesirable under cutting. In this method, a gas such as CF4 is injected into the chamber which contains the wafer to be etched. The chamber is maintained at a relative vacuum and the gas is converted into a plasma by the coupling of the chamber to an R.F. frequency power source. This creates radicals which are chemically reactive with the surface to be etched, thus removing the desired material which is continually removed from the chamber. Like all methods, it is important to detect when the desired overlying material has been completely removed. The sensing of the complete etching of a layer of material has been carried out in the past using any of several techniques. One such technique involves monitoring the plasma emission during etching, at a particular wavelength, and the intensity at that wavelength is then correlated to the remaining thickness of a film being etched. In this manner, it can be determined when one or more known film thicknesses remain over a substrate. Another technique involves spectroscopic monitoring of the substrate to determine when the atomic lines of certain elements, such as phosphorus disappear, as in the case of etching phosphorus-doped silicon dioxide. Still another technique for monitoring etch depth relies on the transparency to a visible light and a light having a wave length from about 400 nm to about 700 nm of substrate layers. Such a layer is transparent to incident visible light if it transmits at least 5% of the incident visible light. In this technique, visible light from, for example, a laser, is directed onto an uncovered area of the transparent layer undergoing etching, and the intensity of the light reflected from the layer is detected and recorded as a junction of time. Because the layer is transparent, the incident light is both reflected from the upper surface of the transparent layer and is transmitted through the layer. As etching proceeds, the thickness of, and thus the optical path length through the substrate layer being etched is reduced. Consequently, as specific thickness, destructive or constructive interference, which correspond to, respectively, a relative minimum and a relative maximum in the recorded density-time curve, occurs. It is possible to relate the time intervals between these intensity extremes to changes in etch depth.
In addition to the problem of monitoring etch depth, to assure that layers are completely etched to the desired level, it is also necessary to monitor wafers following etching to determine whether any contaminants remain on the wafer following the etching process. For example, one type of plasma etching relies on the use of a nitride to perform the etching task. This process, sometimes referred to as nitride etching, may, in some cases result in nitride residue to remaining on the surface of a wafer that causes nitride etching non-uniformity. This nitride residue represents a contaminant which has a damaging affect on the wafer when the wafer is subjected to subsequent processing steps. In the past, there has been no efficient means to detect the presence of nitride residue in the wafers after they exit the plasma chamber, before being transported to a subsequent processing station. As a result, the presence of undetected nitride residue on semiconductor wafers has caused the production of defective product, which in tun reduces yield and productivity.
Accordingly, their is a clear need in the art for a method and apparatus for detecting the presence of a nitride residue on the surface of a wafer immediately after it has been removed from the etching chamber.
According to one aspect of the invention, a method is provided for detecting the presence of nitride residue on the surface of a semiconductor wafer after a layer of material on a wafer has been etched using a plasma etching technique. The method includes the steps of subjecting the wafer surface to light; measuring the magnitude of light reflected from the wafer surface, having a wavelength within a range of wavelengths characterizing the color of the nitride residue; and comparing the measured magnitude with a reference value, the result of such comparison indicating the presence or absence of nitride residue on the wafer surface. In the case of a nitride etchant, the wavelength range of light is characteristic of the color purple. The light is preferably directed on to the surface of the wafer using a pair of color analyzers which also sense the light reflected from the wafer and analyzes its wavelength.
According to another aspect of the invention, a method is provided for etching a layer of material on the surface of a semiconductor wafer, which includes the steps of; transferring the wafer into a processing chamber; plasma etching a layer on the wafer using an etchant including a nitride; transferring the wafer out of the chamber after etching is complete; directing light onto the surface of the wafer after the wafer has been transferred out of the chamber; and measuring the light reflected from the wafer surface having a wavelength within a range of wavelengths that characterize nitride residue remaining on the wafer surface.
According to a further aspect of the invention, apparatus is provided for detecting the presence of nitride residue present on the surface of a semiconductor wafer, after the wafer has been removed from a chamber in which the wafer is etched. The apparatus includes a color differential sensor system for sensing and analyzing light reflected from the fully etched areas of the wafer surface and from areas containing the residue, the sensed differential indicating the presence or absence of the residue. The color differential sensor preferably includes first and second color sensors mounted so as to sense the color of light reflected from diametrically opposite sides of the wafer surface. The apparatus also includes means responsive to the sensor system for controlling the transfer of the wafer.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for sensing the presence of a nitride residue on the surface semiconductor wafer after the wafer has been plasma etched.
A further object of the invention is to provide a method and apparatus as described above which detects the presence of nitride residue on the wafer surface immediately after the wafer exits an etch chamber, and before the wafer is transferred to a subsequent processing station.
Another object of the invention is to provide a method and apparatus as aforementioned which materially reduces scrap and increases yield by eliminating damaging contaminants on the wafer surface.
A further object of the present invention is to provide a method and apparatus of the type mentioned above which automates the process of inspecting wafers for nitride residue, thus increasing efficiency and reducing the possibility of inspection error.
These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of the preferred embodiment of the invention.